A partial discharge is a small electrical spark which is caused by deterioration of the electrical insulation used in high voltage electrical equipment such as motors and generators. The electrical pulses propagated by partial discharges can be used to indicate deteriorating insulation in the stator winding insulation, and to some degree the extent of the deterioration, which will eventually lead to short circuiting and stator winding failure. Thus, through detection of partial discharges motor and generator users can determine whether stator winding failure is imminent, and take corrective action accordingly.
Partial discharges also occur in power cables, gas insulated switchgear, and other equipment where high voltage electrical insulation is subject to deterioration. The partial discharge detector of the present invention can be used to distinguish between partial discharge pulses and pulses from other sources in all such equipment.
There are many methods of measuring partial discharge. Most involve detecting the electrical signals which accompany a partial discharge within deteriorating insulation. Sensors which detect partial discharge signals include capacitors, current transformers and high frequency electromagnetic antennae. Signals from such sensors are displayed in the time domain on an oscilloscope, or in the frequency domain on a frequency spectrum analyzer, and are analyzed by an expert to determine whether the signal does in fact represent a partial discharge.
U.S. Pat. No. 4,949,001 issued Aug. 14, 1990, which is incorporated herein by reference, teaches an instrument, referred to as a "stator slot coupler" ("SCC"), which allows an operator to determine the nature and location of partial discharge within a stator winding with considerably more accuracy than afforded by previous techniques. However, the results still must be interpreted by an experienced operator, because in a typical plant operating motors and generators there is considerable electrical interference (noise) generated by power tools, arcing carbon brushes on slip rings, high voltage switchgear and transformers and switching noise from electronic power supplies, all of which obscure the relatively small partial discharge pulses from the winding. The need for an expert to observe significantly increases the cost of performing such a test, so users tend to test their equipment much less frequently than they should. Ideally motors and generators should be tested for stator winding deterioration at least semiannually, an interval at which any increase in partial discharge activity will be detected before winding failure occurs.
Many types of noise have a frequency content differing significantly from that of a partial discharge pulse, and previous attempts to develop devices which distinguish between partial discharge and noise have relied on filtering techniques which discriminate between differing frequencies. However, some types of noise, such as corona from high voltage switchgear, have essentially the same frequency content as stator winding partial discharge. Thus, filtering is not always a reliable method of eliminating noise.
Another technique is to block the noise signal from reaching the recording instrument, using signals from auxiliary sensors located close to the sources of noise. The equipment is designed to ignore pulses received at the primary sensors if the pulses are close in time to pulses received at the auxiliary sensors, since such pulses are known to be noise, and these pulses are thus not transmitted to the recording instrument. However, in practical applications in an industrial plant it is often not possible to anticipate or locate all noise sources, so the results of partial discharge testing may not be reliable in any particular case.
More recently pattern recognition techniques based on statistical analysis and neural networks have been applied. Pulses are recorded, and displayed in multi-dimensional graphs of number of pulses versus magnitude versus phase position, and post-processing software is used to find patterns associated with noise and partial discharge. These techniques require prior characterization of the noise environment by an expert, and cannot be adapted to changes in the noise environment over time.
Another technique applicable to hydrogenerator stator windings uses a pair of sensors (called PDA couplers) for each phase, installed in specific locations of the stator winding of a particular generator known as a hydrogenerator. Based on the principle that it takes time for partial discharge and noise pulses to propagate through the winding, and that these pulses have a maximum rise time of 5 ns, noise can be distinguished from partial discharge based on the arrival time of a pulse at the PDA couplers. This technique works well for hydrogenerators, having internal phase buses which are often longer than 10 m (which corresponds to a pulse travel time of about 30 ns), allowing time for the electronic circuits to react to differences in pulse travel times, but is inadequate for high speed motors and turbine generators where internal phase buses, if there are any, are often less than 1 m in length.
The "stator slot coupler" described in U.S. Pat. No. 4,949,001 is an electromagnetic sensor specifically designed to detect partial discharge activity in operating turbine generators and motors, where PDA couplers are ineffective. The device has a bandwidth of 1000 MHz and is installed within the stator winding slots. The inventors have determined through the use of the stator slot coupler that partial discharge pulses in the immediate environment of the device are single, unipolar pulses having a pulse width of 8 ns or less, whereas all pulses known to originate as noise have a pulse width of 24 ns or greater.
The stator slot coupler is further designed to provide an output from each end of the device, which has a length of about 50 cm. Since a pulse travels through a stator winding at a specific speed (about 0.15 m/ns), by comparing the relative arrival time of the pulse at each end of the stator slot coupler it is possible to determine the direction of the origin of a partial discharge pulse or whether the pulse originated underneath the device. This allows one to determine whether the partial discharge activity is occurring within the stator slot or in the endwinding (outside of the stator slot), and corrective procedures for the stator winding can be designed accordingly. The different arrival times similarly permit a determination of the location of partial discharge in power cables and gas-insulated switchgear.
Previously the preferred instrument for observing the pulses detected by a stator slot coupler was a 1 GHz sample rate or faster, dual channel oscilloscope. However, since there are thousands of partial discharge and noise pulses occurring every second, and commercially available oscilloscopes can only record and display about 10 pulses per second, only a small proportion of such pulses are captured and a considerable amount of information is thus lost.
An instrument capable of separating noise from partial discharge based on the shape (i.e. width) of the pulses, which is also capable of determining the location of the source of the pulse based on the relative arrival time of pulses at each end of the stator slot coupler, cannot use conventional electronic circuitry for a number of reasons. First, the duration of some pulses is extremely small; pulses may be as short as 2 ns. Very fast electronic circuitry would be required to capture the width of such short pulses, and to determine at which end of the sensor the pulses arrived first, when the time difference between sensing of the pulse at each end of the sensor can be as short as 2 ns.
It is desirable to capture and store the width of each pulse because the physics of the partial discharge process suggests that there is probably a correlation between the width of the pulse and the nature of the deterioration. For example, delamination partial discharge may tend to produce pulses of relatively greater width. Moreover, because high frequency pulses attenuate quickly in stator windings and the like, by the time such pulses reach the sensor they may be considerably wider than when initially produced. Thus, it is desirable to be able to measure a range of pulse widths, as opposed to merely distinguishing between pulses less than 8 ns and those greater than 24 ns. It is also desirable to be able to change the characterization of noise and partial discharge pulses, for example to define a partial discharge pulse as having a width of less than 4 ns rather than 8 ns, to accommodate new research findings on the correlation of pulse width with winding deterioration, without any major change in electronic circuitry.
To accomplish all of this requires extremely fast circuitry and many different types of conditions must be decoded. It is not economically feasible to use cryogenic or superconducting electronic components, so conventional components must be used. Moreover, the flash analog-to-digital converters used in digital oscilloscopes which capture the entire waveform of a pulse would result in most partial discharge pulses being ignored since the time required to capture, store and display the pulse shape in a computer would be many milliseconds, whereas the time between partial discharge pulses can be infinitesimal.
The present invention overcomes these disadvantages by providing a method and device for distinguishing between partial discharge pulses and noise pulses. The invention employs comparators to establish upper and lower dc voltage thresholds defining a range of voltages, and produce a digital output depending upon the magnitude of the pulse. The invention uses varying time sampling to measure the pulse magnitude at short and then relatively longer intervals, which significantly reduces decoding and memory requirements. All pulses are sampled and characterized by pulse width, polarity and arrival time, and this information is stored and sorted conventionally to provide a pulse profile, in real time, for signals detected by the sensor. Decoding in real time ensures that, over time, no partial discharge pulse shapes go undetected.
Moreover, the invention accomplishes this using conventional electronic circuitry. By separating the operations of decoding pulse shape and decoding polarity/arrival time/magnitude, relatively inexpensive integrated circuits can be used in the device of the invention. Decoding the pulse type as a .function of time allows flexibility in the definition of partial discharge; thus, in other applications (for example analyzing partial discharge in gas insulated switchgear) where partial discharge pulses may be known to have a shorter pulse width, the definition can be easily changed to accommodate the specific application. The invention can be utilized with any partial discharge sensor capable of receiving wide-band pulses.